Variable gain amplifier circuits are commonly used in the electronics and semiconductor industry for automatic gain control applications, voltage control filters, automatic signal levelling for A/D amplitude modulation and variable gain transimpedance, for example. Commercial devices in which variable gain circuits are used extensively include personal digital assistants (PDAs), mobile communication devices, cellular phones, and wireless two-way data communications devices, collectively referred to herein as mobile devices.
FIG. 1 shows a circuit schematic of a typical variable gain amplifier circuit. Variable gain amplifier (VGA) circuit 10 includes a load stage 12, a differential input stage 14, a degeneration stage 16 and a current source 18 connected in series between a first voltage supply such as VDD and a second voltage supply such as VSS or ground. A first current branch of amplifier circuit 10 consists of resistor 20, n-type bipolar transistor 24 and n-type degeneration transistor 28. A second current branch of amplifier circuit 10 consists of resistor 22, n-type bipolar transistor 26 and n-type degeneration transistor 30, where the resistors, bipolar transistors and degeneration transistors are identical to each other. VGA circuit 10 receives a pair of input signals IN+ and IN− at the base terminal of bipolar transistors 24 and 26 respectively for providing a pair of output signals OUT− and OUT+ at the collector terminals of bipolar transistors 24 and 26 respectively. A degeneration voltage signal DCTRL is connected to both gate terminals of degeneration transistors 28 and 30 for adjusting their effective channel resistance. A bias voltage VBIAS connected to the gate of n-type transistor 18 controls the total current flowing through VGA circuit 10. The voltage gain of the output signals OUT− and OUT+ relative to the input signals IN+ and IN− depends on the ratio of the load resistance and the resistance of the degeneration transistors 28 and 30. More specifically, VG=Rload/Rdegen, where VG is the voltage gain, Rload is the load resistor value and Rdegen is the resistance value of the degeneration transistors. Therefore, by controlling the gate to source voltage of the degeneration transistors 28 and 30 via DCTRL, their effective channel resistance Rdegen can be changed, and thus the gain of VGA circuit 10 can be varied. Although not shown in FIG. 1, a gain control circuit sets the desired gain of VGA circuit 10 through signal DCTRL.
An important and practical criteria of VGA circuit 10 is its ability to provide a wide gain range. Therefore it follows that the resistance of the degeneration transistors 28 and 30 must vary over a wide range. However, design requirements demand that the input linearity of the VGA circuit improve as the gain is lowered. Hence, if the circuit is biased at a fixed tail current by transistor 18 via VBIAS, the range of degeneration resistance is severely limited by bias considerations. If the tail current is kept constant, i.e Vbias=constant, then low gain is achieved by lowering DCTRL to increase Rdegen. A problem with this solution is that the large voltage drop across Rdegen cause voltage headroom limitations for the typical low supply voltages that are used in low power applications. More specifically, as the resistance of the degeneration transistors 28 and 30 increases, the voltage drop across them increases proportionally. Eventually the bias conditions of bipolar transistors 24 and 26 of the differential input stage 14 will no longer be satisfied, and as a result, will begin to turn off. Furthermore, degeneration transistors 28 and 30 dissipate relatively high amounts of power during low gain operation since the effective degeneration transistor resistance and current through the constant current source transistor is high. Since most VGA circuits operate normally at gain levels significantly lower than maximum gain, high power consumption results. Thus, gain of the circuit can be controlled over a wide input signal dynamic range, but at the expense of linearity and/or power, making the VGA circuit 10 prior amplifier control techniques an unacceptable solution in low power/high linearity applications.
It is, therefore, desirable to provide a VGA circuit and controller that provides gain control with high linearity and low power consumption without sacrificing dynamic range.